1. Field of the Invention
The present invention generally relates to an image sensing method, in particular, to a resetting method and an image sensing device applicable to a complementary metal-oxide-semiconductor (CMOS) image sensor.
2. Description of Related Art
As semiconductor and photoelectric techniques develop, recently, more and more digital cameras, digital monitors, or digital video cameras exist in our life. The digital camera, the digital monitor, the digital video camera, or other photographing devices need photosensitive element to convert sensed light signal to electrical signal for output. Generally, the photosensitive element is mostly a charge coupled device (CCD) or a CMOS image sensor. Although the image quality of the CCD is better than the photographing device of the CMOS image sensor, cost of the CCD is higher than the cost of the CMOS image sensor, so many electronic photographing devices still adopt the CMOS image sensor.
The image sensor usually has image lag, and the image lag and dark current often affect with each other. Manufacturer of the image sensor can change process parameters, for example, change characteristics of the transfer gate, so as to adjust the image lag and the dark current of the image sensor, but it is impossible to modify two parameters at the same time. When it intends to reduce the image lag, the dark current is increased; on the contrary, when it intends to reduce the dark current, the image lag is deteriorated. In brief, the image lag and the dark current are of a trade-off relation.
In a conventional method, a voltage pump is used to modify the image lag without increasing the dark current. If the image lag is modified in the process, it is possible to reduce the dark current of the image sensor by using a negative voltage pump. If the dark current is modified in the process, it is possible to reduce the dark current of the image sensor by using a positive voltage pump. However, the cost and the power consumption are increased along with the utilization of the voltage pump, in addition, noises such as random noise, ripple noise, and substrate noise may also increase.
Referring to FIG. 1, a circuit diagram of a conventional CMOS pixel array 10 is shown. The CMOS pixel array 10 includes (N+1)*(M+1) CMOS image sensing units 20. M+1 CMOS image sensing units 20 of the ith row are controlled by a reset signal RST′[i], a control signal TX′[i], and a selecting signal SELECT′[i], in which i is an integer from 0 to N. Firstly, when the reset signal RST′[i] is in a high level, the control signal TX′[i] is in a high level, and the selecting signal SELECT′[i] is in a low level, the M+1 CMOS image sensing units 20 of the ith row are reset. Next, when the reset signal RST′[i] is in a high level, the control signal TX′[i] is in a low level, and the selecting signal SELECT′[i] is in high level, the M+1 CMOS image sensing units 20 of the ith row output signal level after resetting. Finally, when the reset signal RST′[i] is in a low level, the control signal TX′[i] is in a high level, and the selecting signal SELECT′[i] is in a high level, the M+1 CMOS image sensing units 20 of the ith row output sensing signals PIX_OUT[M:0] after exposing, in which PIX_OUT[M:0] represents all the signals of PIX_OUT[0]-PIX_OUT[M].
Referring to FIG. 2, a circuit diagram of the CMOS image sensing unit 20 is shown. As shown in FIG. 2, the CMOS image sensing unit 20 includes a photodiode 200, and transistors 201, 202, 203, and 204. The transistor 201 is coupled to the photodiode 200, the transistor 202 is coupled to the transistor 201, the transistor 203 is coupled to the transistor 202, and the transistor 204 is coupled to the transistor 203. The photodiode 200 is used for exposing, so as to acquire the sensing signal, the transistor 201 is controlled by the control signal TX′[i], the transistor 202 is controlled by the reset signal RST′[i], the transistor 204 is controlled by the selecting signal SELECT′[i], and the transistor 204 is used for outputting the sensing signal PIX_OUT[j] and the signal level after resetting, in which j is an integer from 0 to M.
Referring to FIG. 3, which is a diagram showing waveforms of the reset signals and the control signals during resetting, exposing, and sampling of the conventional CMOS pixel array 10. At time Ti (the scope of i is the same as the above scope, that is, i is an integer between 0 and N), RST′[i]=High and TX′[i]=High (SELECT′[i]=Low, and SELECT′[i] is not shown in FIG. 3), and the M+1 CMOS image sensing units 20 of the ith row are reset. However, from time Ti to T′i, the M+1 CMOS image sensing units 20 of the ith row expose. At time T′i, RST′[i]=High and TX′[i]=Low (SELECT′[i]=High), and the M+1 CMOS image sensing units 20 of the ith row output the signal level after resetting. Finally, at time S′i, RST′[i]=Low and TX′[i]=High (SELECT′[i]=High), and the M+1 CMOS image sensing units 20 of the ith row output the sensing signals PIX_OUT[M:0] acquired by exposing.
In FIG. 3, exposing time required by the M+1 CMOS image sensing units 20 of each row is the same, and generally, the time required by resetting cannot be too long, approximately between 1 and 10 μs. Therefore, in order to prevent the image lag, the above described voltage pump is used to accelerate the resetting speed.
The cost and the power consumption increase when the voltage pump is used, in addition, the noises such as random noise, ripple noise, and substrate noise also increase. Therefore, in order to reduce the noises and to prevent the image lag, the present invention provides a method for resetting an image sensor by using block as a unit and an image sensing device using the same.